This application claims the priority of Japanese Patent Application No. 2001-81102 filed on Mar. 21, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a wide-angle three-group zoom lens, and more particularly, to a zoom lens which is suitable for use with a single-lens reflex (SLR) camera requiring a long back focal length, enables achievement of an angle of view of about 70 degrees at the wide-angle end thereof and a variable power ratio of about 2.2, and is composed of three groups.
2. Description of the Prior Art
There have hitherto been known various types of wide-angle zoom lenses which can realize a long back focal length required when the lens is provided in an SLR or the like and which involve lens arrangement of negative, positive, and negative.
Such zoom lenses include zoom lenses described in, e.g., Japanese Patent Publication No. 6(1994)-40170 and Japanese Unexamined Patent Publication No. 10(1998)-26727.
The zoom lenses described in the publications have lens system whose overall lengths are comparatively small and have achieved a zoom ratio of about 2.5.
However, each of the zoom lenses described in the publication suffers from an increase in the diameter of an object-side lens (front lens) in a first group of lenses. As a result, miniaturization of the overall lens system and a lens barrel is hindered, thereby posing difficulty in increasing the speed of zooming response.
The present invention has been conceived in view of the above circumstances and aims at providing a wide-angle three-group zoom lens which assures a required back focal length, a reduction in overall length, and miniaturization of a lens system by means of reducing the diameter of a front lens while maintaining superior optical performance.
The present invention provides a wide-angle three-group zoom lens, comprising a first group of lenses having negative refracting power, a second group of lenses having positive refracting power, and a third group of lenses having positive refracting power, which are provided in sequence from an object side of the zoom lens, wherein zooming is effected by changing intervals between the lens groups; and wherein the second group of lenses further comprises, in sequence from the object side, a biconvex lens group 2A, a lens group 2B formed by cementing two or more lenses together, an aperture, a lens group 2C formed by cementing together two or more lenses, and a lens group 2D which has positive refracting power and whose convex side is oriented toward the image surface side.
Preferably, the third group of lenses is composed of two lenses; that is, a negative lens 3A whose concave side is oriented toward the object, and a positive lens 3B whose convex surface is oriented toward an image surface side of the zoom lens.
Preferably, the first group of lenses comprises, in sequence from the object side, five lenses; that is, a lens 1A having the shape of a negative meniscus and whose convex side is oriented toward the object; a lens 1B having the shape of a negative meniscus and whose convex side is oriented toward the object; a cemented lens formed from a negative lens 1C whose concave side is oriented toward the object and a positive lens 1D; and a positive lens 1E whose convex side is oriented toward the object.
Preferably, the lens group 2C is constituted by means of cementing two lenses together. More preferably, the lens group 2B is constituted by means of cementing three lenses together.
The present invention provides a wide-angle three-group zoom lens, comprising a first group of lenses having negative refracting power, a second group of lenses having positive refracting power, and a third group of lenses having positive refracting power, which are provided in sequence from an object side of the zoom lens, wherein zooming is effected by changing intervals between the lens groups; the second group of lenses further comprises, in sequence from the object side, a biconvex lens group 2A, a lens group 2B formed by cementing three lenses together, an aperture, a lens group 2C formed by cementing together two lenses, and a lens group 2D which has positive refracting power and whose convex side is oriented toward the image surface side; wherein conditional equation (1) provided below is satisfied, provided that a combined focal length of the lens group 2A and the lens group 2B is set to f2F and that a combined focal length of the lens group 2C and the lens group 2D is set as f2R.                               -          0.7                 less than                               f                          2              ⁢              F                                            f                          2              ⁢              R                                       less than                   +          0.1                                    (        1        )            
Preferably, conditional equation (2) provided below is satisfied, provided that a radius of curvature of an object-side cementing surface of the lens group 2B is taken as R22F and an image-surface-side cementing surface of the lens group 2B is taken as R22R.                               -          1.5                 less than                               R                          22              ⁢              R                                            R                          22              ⁢              F                                       less than                   +          0.1                                    (        2        )            